Both experimental animals and humans can regain the ability to stand or to step after a complete spinal cord transection. The ability to execute these tasks depends on specific training regimens, illustrating the importance of motor leading in the spinal cord. Low-thoracic transection and subsequent training leads to a persistent increase in the total inhibitory capacity of the lumbar spinal cord, exhibited by increases in GAD67, a GABA-synthesizing enzyme, and its mRNA, as well as in the alpha-1 subunit of glycine receptor and in gephyrin, a protein associated with glycine receptors. However, repetitive hindlimb training, such as stepping, returns the levels of GAD67 and glycine receptors towards normal. The central hypothesis of this proposal is that task-specific, repetitive training selectively modulates the inhibition within sensorimotor pathways associated with the execution of that task. Using a robotic device, we will test this hypothesis with a well-defined standing task in neonatally transected (T12-13) rats. Stand training allows us to compare training-induced changes in neurons associated with plantarflexion (facilitation of the soleus motor pool) and neurons associated with dorsiflexion (inhibition of the tibialis anterior motor pool). We will test three hypotheses: (1) that stand training decreases the inhibitory capacity of specific neurons associated with the ankle dorsiflexor (tibialis anterior); (2) that training selectively alters the ratio of inhibitory and excitatory synapses on the somata of individual motoneurons in motor pools associated with soleus, and (3) that pharmacologically induced changes in motor performance of spinally transected rats reflect these alterations in GABAergic and glycinergic inhibition in plantarflexion- and dorsiflexor-associated neurons as noted in the first two hypotheses. A major innovation in this work is the ability to train motor tasks, and to quantify the kinematics of standing and stepping using a newly developed robotic device. This device will allow us to impose strictly repetitive training and to assess the progress of individual animals with great precision. The proposed studies address the anatomical and molecular bases of the plasticity that may underlie rehabilitative training after spinal injury. This work will lead to better ways of testing the effectiveness of alternative training strategies and associated pharmacological interventions.